Inductive heating installations for flat objects are either of the longitudinal field heating type or of the transversal field heating type. In the case of longitudinal field heating, the metal sheet to be heated is completely enclosed by the inductor coil, so that the main magnetic flux is directed in the advancing direction of the sheet. The induced currents follow a continuous path over the cross section of the workpiece, the resultant temperature distribution at a suitable frequency of the inductor current being virtually homogeneous over the entire strip width of the flat object. A disadvantage is the high required frequency and limited efficiency.
This was the reason for the development of transversal field heating installations, in which the induction coils do not enclose the flat object but are arranged on the surfaces of the flat object that is to be heated. In this way, the main magnetic flux of the induction coil is directed perpendicularly in relation to the surface of the flat object. However, the disadvantage of transversal field heating installations is that the temperature distribution in the flat object is normally inhomogeneous. This requires an adapted design of the geometrical dimensions and an optimization of the operating parameters of a transversal field heating installation for each case in which it is used.
In A. Ruhnke, A Mühlbauer, A. Nikanorov, V. Demidovitch, “Wege zur Optimierung von Querfeld-Erwärmungsanlagen” [Ways of optimizing transversal field heating installations], Elektrowärme International, issue B4, December 1997, pages 130 to 137, it is described that the disadvantageous inhomogeneous temperature distribution can be influenced by specific configuration of the induction coil. In the design of transversal field heating installations, account must be taken off the complex three-dimensional distribution of the electromagnetic field variables that are not analytically determinable. Moreover, numerous operating and installation parameters must be predicted and taken into account. A major influencing factor here is the strip overhang. This is the distance between the outer edge of the end winding of the inductive coil and the strip edge of the flat object. Furthermore, it is described that the installations can be adapted more flexibly to the respective requirements with the aid of multi-turn coils, in that the geometry of the end windings is varied by a spread distribution of the individual conductors.
For flexible setting of the temperature distribution in the flat object, an inductor loop comprising two induction coils transversely in relation to the flat object and two induction coils longitudinally in relation to the flat object is described in Japanese Patent Application 63195397. The two coils extending in the longitudinal direction of the flat object can be displaced in the transverse direction of the flat object, so that the temperature distribution at the side edges of the flat object can be set.
Furthermore, DE 39 28 629 A1 discloses the use of an inductor coil for induction furnaces in which grooves of an iron core formed by transformer sheets are let in. The iron cores are formed in a zigzag or undulating manner and the current conductors are switchable, so that the installation can be set flexibly to the respective width of the flat object.
Furthermore, EP 0 667 731 B1 discloses a transversal field heating assembly with an adjustable width for induction heating, in which two halves of an inductor coil can be displaced parallel to each other, so that the distance of the end windings from one another can be changed. In this way, the inductor unit can be flexibly adapted to the respective width of the flat object.
Furthermore, in DE 42 34 406 A1 there is described a transversal field sheet heating installation in which the inductor coils are arranged in the advancing direction of the flat object in such a way that each edge of the flat object is projected over by the end windings of only one induction loop. The end windings consequently end alternately in the region of the side edge on the right and the side edge on the left of the flat object and do not protrude beyond the side edge.
The problem of the conventional transversal field heating installations is that it is only possible with great effort for them to be adapted to the respective operating situation, in particular the strip width. Moreover, the end winding, which is important for the temperature distribution, cannot be varied at the edges of the strip in conventional transversal field heating installations.
The end winding is important in this way because, as described for example in DE 100 13 061 A1, optimal configuration of it allows the temperature distribution to be made considerably more uniform, in particular at the strip edges.